This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. The goal of this project is to determine the role that the cell types of the outer retina play in light adaptation, color-coding and receptive field structure. Over the last year the project has focused on the role of the H1 horizontal cell[unreadable]a key interneuron[unreadable]by characterizing sensitivity regulation, receptive field structure and cone input. First, we have made significant progress in understanding how the spatial receptive field of the H1 horizontal cell contributes to the surrounds of inner retinal neurons. The primate H1 horizontal cells are coupled to each other electrically by gap junctions and access the cone-bipolar-ganglion cell pathway by feedback at the cone-H1 synapse. We have determined how direct synaptic input and coupled input interact to form a receptive field and have measured the relative contributions of these components using pharmacology and neural simulations. In the fovea, coupling is reduced and the receptive field size of H1 cells shrinks to create a surround derived from only a handful of cones. These results suggest strongly that this pathway is a critical component of the foveal 'midget'red-green color-opponent pathway. Finally, as part of an ongoing study of sensitivity regulation in the outer retina we have characterized how H1 and H2 horizontal cells contribute to retinal light adaptation by measuring the way in which signals from L, M and S cones are desensitized over the photopic range of light adaptation. From these results we have concluded that the first retinal synapse between H1 cells and photoreceptors is a major site for sensitivity regulation in outer retina.